(1) Field of the Invention
The present invention relates to a manufacturing method of a display device, in particular, to an effective technology that is applicable to a manufacturing method of an active matrix display device having a drive circuit formed outside a display area.
(2) Description of the Related Art
Among liquid crystal display devices have been an active matrix liquid crystal display devices. An active matrix liquid crystal display device has multiple active elements (also called “switching elements”) arranged in the display area of a liquid crystal display panel in a matrix. Used as the active elements are, for example, thin film transistors (TFTs). In the liquid crystal display panel, each TFT basically has a metal insulator semiconductor (MIS) structure including a metal layer, an insulating layer, and a semiconductor layer. Mainly used as the semiconductor layer of each TFT is silicon (Si). If the insulator layer is made of, for example, silicon oxide film (SiO2), the TFT structure is called a metal oxide semiconductor (MOS).
In conventional liquid crystal display devices, a chip-shaped driver integrated circuit (IC) has mainly been used as a drive circuit for generating a signal voltage to be applied to scan signal lines and video signal lines of a liquid crystal display panel, controlling the timing at which the voltage is applied, and doing other things. Such a driver IC, for example, may be packaged directly on a substrate (hereinafter referred to as a “TFT substrate”) on which active elements (TFTs) are arranged in a matrix, in the display area of a liquid crystal display panel. A tape carrier package (TCP), a chip on film (COF), or the like, on which a driver IC is packaged, may be coupled to the TFT substrate. In recent years, there has also been proposed a liquid crystal display device in which a drive circuit that uses a TFT or the like to function as the driver IC does is formed outside the display area of the TFT substrate so as to be integral with the TFT substrate.
The drive circuit formed integrally with the TFT substrate is an IC including multiple TFT (MOS transistor) diodes, a capacitor, a resistance, and the like. The drive circuit can be formed in the process of forming scan signal lines, video signal lines, active elements (TFTS), and the like on the TFT substrate, together with these components.
If the drive circuit is formed outside the display area of the TFT substrate, an n-channel MOS transistor (hereafter referred to as an “nMOS”) and a p-channel MOS transistor (hereafter referred to as a “pMOS”) may be formed. In this case, the channel region of the nMOS semiconductor layer must be made a p-type or n-type semiconductor region, and the drain and source regions thereof must be made n-type semiconductor regions. The channel region of the pMOS semiconductor layer must be a p-type or n-type semiconductor region, and the drain and source regions thereof must be made p-type semiconductor regions. If the channel region of the nMOS is made an n-type semiconductor region, its donor concentration must be different from those of the drain and source regions. Similarly, if the channel region of the pMOS is made a p-type semiconductor region, its acceptor concentration must be different from those of the drain and source regions.
If a semiconductor layer (Si layer) is made a p-type semiconductor, it is a common practice to form a resist having an opening only in a region that is desired to be made a p-type semiconductor and then to ion-implant an impurity (acceptor) such as a boron ion (B+) to the semiconductor layer. Similarly, if a semiconductor layer (Si layer) is made an n-type semiconductor, it is a common practice to form a resist having an opening only in a region that is desired to be made an n-type semiconductor and then to ion-implant an impurity (donor) such as a phosphorus ion (P+) to the semiconductor layer. In this case, each resist is formed, for example, by applying a photosensitive resist material to the semiconductor layer, then exposing the resist material using an exposure mask, and develop the resist material. Thus, the related art impurity implantation method has had not only a problem that there is an increase in the number of exposure masks necessary to partition the semiconductor layer of the nMOS into semiconductor regions as well as to partition the semiconductor layer of the pMOS into semiconductor regions, but also a problem that there is an increase in the frequency of exposure as well as in that of development.
However, for example, the following method has been proposed in recent years as a method of reducing the number of exposure masks necessary to make semiconductor layers an n-type semiconductor(s) and a p-type semiconductor(s). In that method, an acceptor (or donor) is implanted to the semiconductor layers to make all the semiconductor layers p-type (or n-type) semiconductors; a resist is formed so as to cover a semiconductor region(s) that is desired to be made a p-type (or n-type) semiconductor and so as to have an opening above a semiconductor layer that is desired to be made an n-type (or p-type) semiconductor; and a donor (or acceptor) is implanted to only the semiconductor layer on which the resist has the opening so that the semiconductor layer is made an n-type (or p-type) semiconductor (for example, see Japanese Patent Application Laid-Open Publication No. 2003-282880). For example, if this method is used to make all the semiconductor layers p-type semiconductors and then to make the semiconductor layer above which the resist has the opening an n-type semiconductor, it is sufficient to make the amount of the donor to be implanted to the semiconductor layer above which the resist has the opening larger than the amount of the already implanted acceptor so that the donor concentration becomes higher than the acceptor concentration. This allows the semiconductor layers to be made an n-type semiconductor and p-type semiconductors using one exposure mask.
With regard to the TFT substrate of a recent liquid crystal display device, when a holding capacitance is formed for each pixel in the display area, a part of the semiconductor layer of an active element (TFT) is sometimes made conductive so as to be used as one electrode of the holding capacitance. If the active element (TFT) is an nMOS, the channel region of the semiconductor layer is made a p-type (or n-type) semiconductor, and the drain and source regions thereof and the part thereof to be used as the electrode of the holding capacitance are made n-type semiconductors. In this case, the carrier concentration (donor concentration) of the part must be approximately equal to those of the drain and source regions.
As a method of forming the semiconductor layer of the TFT if a part of the semiconductor layer is used as the electrode of the holding capacitance, for example, the following method has been proposed. In that method, an impurity (acceptor) is implanted to the entire semiconductor layer, including the part thereof to be used as an electrode of the holding capacitance, so as to make the semiconductor layer a p-type semiconductor; an impurity (donor) is implanted to only the part to be used as the electrode of the holding capacitance and the part's proximity so as to make them an n-type semiconductor; scan signal lines and holding capacitance lines are formed; and an impurity (donor) is implanted to the drain and source regions of the semiconductor layer with these lines used as masks so as to make these regions n-type semiconductors (for example, see Japanese Patent Application Laid-Open Publication No. 2005-274984).
However, if the drive circuit is formed outside the display area of the TFT substrate, for example, an nMOS and a pMOS, and a holding capacitance that uses as an electrode thereof a part of the semiconductor layer of an active element in the display area are sometimes formed on the TFT substrate.
In this case, even if the method described in Japanese Patent Application Laid-Open Publication No. 2003-282880 or Japanese Patent Application Laid-Open Publication No. 2005-274984 is used, for example, an exposure mask for making the semiconductor layers an n-type semiconductor(s) and a p-type semiconductor(s) and an exposure mask for making two types of n-type semiconductors that are different in donor concentration are required. Thus, the number of exposure masks to be created is increased, resulting in a problem of an increase in the manufacturing cost of the TFT substrate (liquid crystal display device).
Further, if the semiconductor layers are made three types of n-type semiconductors that are different in donor concentration, at least two exposure masks are also required in the related art method. Thus, the number of exposure masks to be created is increased, resulting in a problem of an increase in the manufacturing cost of the TFT substrate (liquid crystal display device).